The present invention pertains to variable gain amplifiers and pertains particularly to a variable gain amplifier with adjustable gain slope.
Signals with a wide range of amplitudes, such as with audio amplification, often require amplifiers with a log-linear or a compression-type response. For example, high quality audio may have a range of 64,000 to 1 or greater, representing a range of 1 millivolt to 64 volts. For many amplifiers 1 millivolt is too noisy, and 64 volts is too high a voltage level. A compression amplifier, for the same range, typically has a much smaller and therefore more manageable set of operating voltages. However, compression type amplifiers have a gain that varies with amplitude, and the output may contain substantial distortion products. Many applications are adversely affected by this distortion, which can be shown to be due to large amounts of harmonics introduced in the process of compressing the signal.
A decibel (dB)-linear amplifier can achieve substantially the same result with much less harmonics by varying the gain as a function of an externally introduced control signal. However, forming a dB-linear amplifier requires that the compression products be separated from the linear signal being amplified. Attempts to solve this problem have been made many times. In one approach dB-linear amplifiers are realized with piecewise-linear approximation techniques. The output of the amplifier is controlled by, for example, feedback that varies with operating point. As the control voltage increases the amplifier gain decreases with an approximate fit to a log-linear curve, e. g., an exponential or square law curve. This provides fairly good operation, but to prevent distortion many steps in the piecewise-linear approximation are required, and the complexity and cost of implementation becomes quite large. This is because the approach approximates a smooth exponential waveform with a series of straight lines, and to gain relative smoothness in the output of the amplifier there must be many small lines.
It is also known that a semiconductor diode exhibits a log-linear response (the natural log), but the range is set by the diode characteristics, which vary with temperature and have an offset that may be difficult to compensate for. Additionally, the same problem of simultaneously varying the gain and the amplitude of the input, forming cross products, still exists. Log-linear amplifiers built with this principle have been reported to be linear and well behaved over more than six decades of operating range, but they are difficult to use due to their characteristics. In fact, piece-wise linear amplifiers are often preferred to avoid the problems with diode log-linear amplifiers.
All semiconductors are temperature sensitive. A technique commonly used in band gap regulators and similar circuits allows the temperature sensitivity to be adjusted, but for the prior art dB-linear amplifiers, the adjustment is performed in a separate step or module, which can lead to multiplying errors between the modules. One approach is to make a temperature stable separate amplifier and apply it to a piece-wise linear amplifier. This further increases the cost and complexity of the amplifier.
In accordance with a preferred embodiment of the present invention, a variable gain amplifier has a gain control circuit. The gain control circuit includes a first control input and a second control input. The first control input receives a first control signal. The gain control circuit varies gain of the variable gain amplifier based on a value of the first control signal. The second control input receives a second control signal. The gain control circuit varies gain slope of the variable gain amplifier based on a value of the second control signal.